Electrophotographic method of imagewise particle transfer employing alternating modulated field

ABSTRACT

A process and a device for electrophotographic reproduction using a photoconductive layer placed in contact with a uniform layer of developer between two electrodes. An alternatively modulated voltage is applied to the electrodes.

Unlted States Patent 11 1 [111 3,776,722

Cantarano 1 Dec. 4, 1973 ELECTROPHOTOGRAPHIC METHOD OF IMAGEWISEPARTICLE TRANSFER [56] References Cited EMPLOYING ALTERNATING MODULATEDED STATES PATENTS FIELD 2,901,348 8/1959 Dessauer et al 96 1 R [76]Inventor: Marcus Cantarano, No. 47, av. F. gloncfieff'geatesm ugarman,r. Roosevelt Thlals France 2,901,374 8/1959 Gundlach 915 14 [22] Filed:June 14, 1971 [21] Appl No; 152,962 Primary Examiner-George F. LesmesRelated US. Application Data Continuation-impart of Ser. No. 715,313,March 22, 1968, abandoned, which is a continuation-in-part of Ser. No.613,792, April 18, 1967, abandoned.

Field of Search 96/1, 1.3, 1.4; ll7/17.5 LE

Assistant ExaminerM. B. Wittenberg AttorneyJohn W. Malley et a].

[57 ABSTRACT A process and a device for electrophotographic reproductionusing a photoconductive layer placed in contact with a unifonn layer ofdeveloper between two electrodes. An alternatively modulated voltage isapplied to the electrodes.

14 Claims, 5 Drawing Figures PATENTEU [IEC 41975 SHEET 1 UF 2 Fig.1

Fig.2

1 ELECTROPHOTOGRAPHIC METHOD OF IMAGEWISE PARTICLE TRANSFER EMPLOYINGALTERNATING MODULATED FIELD This application is a continuation-in-partof my application Ser. No. 715,313, filed Mar. 22, 1968 now abandonedwhich in turn is a continuation-in-part of my application Ser. No.613,792, filed Apr. 18, 1967 now abandoned.

This invention relates to the production of electrographic images from alight image forming a conductivity pattern in a layer of aphotoconductive material.

As used herein, the term conductivity pattern is to be understood asincluding any virtually plane surface formed by parts having differentelectric conductivities.

in accordance with prior art, the term insulating is to be understood asdefining the quality of having an electric conductivity lower thanl'Siemens/cm and the term non-insulating as defining the quality ofhaving an electric conductivity superior to Siemens/cm.

In the actual art, a feature of electrographic methods resides in theuse of an original provided with a conductivity pattern including highinsulating parts which will selectively hold electric charges to form alatent electrostatic image; thus an electrographic image may bedeveloped byan electrically responsive powder which adheres to thecharges parts of the latent image. This electrographic image will not beobtained in a stable way because of the passage of electric charges eventhrough the high insulating parts of the original, which would cause theeffacement of at least a part of the latent image during the step of thedevelopment. A typical original of actual electrography consists in aphotoconductive insulating layer provided with a conductivity patternresulting from an exposure to a light image; such a photoconductivelayer will be a high insulator in the dark in order to obtain aconductivity pattern including the non-illuminated high insulating partsserving to develop an electrographic image according to existingmethods. These photoconductive insulating layers are slow in theirresponse to successive different exposures to the light and,consequently, they may not be used to afford high speed processes toproduce successive different electrographic images. Furthermore, theseinsulating layershaving a very low sensitivity to the light, theenlarging ofa document is still difficult to obtain in electrography.

l have found, however, that a stable electrographic image may be formedand simultaneously developed from any original provided with a patternof conductive and low conductive parts in the absence of a latentelectrostatic image; to this end an alternating electric field isgenerated to charge a thin layer of developer powder from theconductivity pattern of the original and thus to apply to this powderelectric charges having different maximum values according to thedifferent conductivities of said pattern. Under the influence of theelectric field, the most charged particles are electrically removed fromthe layer of powder, while a stable electrographic image is developed bythe remaining part of the powder which is never sufficiently charged tobe removed. One of such form of electrographic method is disclosed in myco-pending application Ser. No. 631,792, filed Apr. 18, 1967, nowabandoned. The present invention thus relates to the formation and thesimultaneous development of a stable electrographic image from anoriginal consisting in a photoconductive layer forming a pattern ofconductive and low conductive parts as a result of an exposure to alight image. This application is a continuation-in-part of applicationSer. No. 631,792, filed Apr. 18, 1967 now abandoned,

Now in accordance with the present invention, it has been found that astable electrographic image may be produced from a light image forming apattern of conductive and low conductive parts ina layer ofphotoconductive material. In the preferred formof the invention aphotoconductive layer is used which .is highly conductive when exposedto the light, the dark conductivity of this layer being not critical toobtain a stable electrographic image of satisfactory quality accordingto the invention. Such a photoconductive layer can be calledphotoconductive non-insulating layer to distin-- guish it from thephotoconductive insulating layers teached by Carlson in the US. Pat. No.2,297,691. The non-insulating layers of the type of the well knownphotoconductive layers used in the photoelectric cells, offer theadvantage of having a virtually instantaneous response and a highsensitivity to the light and thus they are well adapted for the highspeed production of stable images and for the electrographic enlargingof documents.

According to one embodiment of the present invention, a thinphotoconductive layer is affixed toan insulating material, thephotoconductive layer is exposed to a light image forming a conductivitypattern in this layer, and an alternating electric field is generated tocharge a thin layer of developer powder from said conductivity patternand thus to apply to the powder electric charges having differentmaximum values according to the different conductivities of saidpattern. Under the influence of the alternating field, the chargedpowder is electrically attracted away from the most conductive parts ofsaid pattern, while the remaining powder is never sufficiently chargedto be removed from the least conductive parts of said pattern and itdevelops a stable electrographic image thereon. The satisfactory qualityof the obtained image is irrespective of a critical duration of theelectric field. This method is well adapted to exactly reproduce thedense large areas and the half-shadow areas of the light image.

According to another embodiment of this invention, the photoconductivelayer is affixed to an insulating backing material and it is coated witha thin layer of developer powder, an insulating layer is-placed againstthe layer of powder, and an electric field is generated to charge thepowder from the conductivity pattern of the photoconductive layer;because of the insulation of the coated photoconductive layer betweenthe insulating backing and the insulating layer, the coating powder willreceive electric charges having maximum values in proportion to theconductivities of said pattern. Accordingly, the charged powder iselectrically attracted away from the most conductive parts of saidtively modulated electric field this method is well adapted to producestable electrographic images from a photoconductive layer exposed to alow contrastful light image. Accordingly, under the action of themodulated field particles of powder receive electric charges ofsuccessive opposite polarities and they are attracted away from the mostconductive parts of said pattern. Because of the opposite charges of thepowder particles, the removal of the powder may be prosecuted toelectrically remove all of the powder coating the most conductive partsof said pattern while the remaining part of the coating powder is neversufficiently charged to be removed from the least conductive parts ofthe layer and thus it forms a stable electrographic image thereon.

According to a further embodiment of the invention, the thin layer ofdeveloper powder is placed against and sandwiched between aphotoconductve layer and an image carrier having a uniform electricconductivity between a photoconductive layer and an image carrier havinga uniform electric conductivity between the maximum and the minimumconductivites of the pattern in the photoconductive layer, and anelectric field is generated to charge the powder from said conductivitypattern and said image carrier. Because of the intermediate conductivityof the image carrier, under the influence of the electric field theparticles of powder are electrically charged under the sign of thatsurface in contact which is the lesser conductive and they are attractedaway from the most conductive parts of said pattern to form a firststable electrographic image on said image carrier, while another part ofthe powder is electrically attracted towards the least conductive partsof said pattern to form a second stable electrographic image thereon.The best quality of the images is obtained by generating an alternatingor an alternatively modulated electric field. This method is welladapted to the high speed development of two simultaneous stableelectrographic images from the same light image.

An object of this invention is to improve electrographic methods and toprovide means and devices for use in electrography.

Other objects of this invention will be apparent from the followingdescription and accompanying drawings taken in connection with theappended claims.

In the drawings:

FIG. 1 is a schematic side elevation view ofa first embodiment for thecarrying out of the invention;

FIG. 2 is a schematic side elevation view of a second embodiment;

' FIG. 3 is a diagram of the forces acting on the powder grains coatingthe photoconductive layer;

FIG. 4 is a second diagram of the forces acting on the powder grainsplaced against the photoconductive layer and the image carrier;

FIG. 5 is a diagrammatic side elevation view of a mechanical deviceaccording to the FIG. 2 embodiment.

In the preferred form of this invention a layer of a photoconductivenon-insulating material is used which has a high sensitivity and avirtually instantaneous response to the light. Alternatively, manypbotoconductive non-insulating materials may be used such as, forexample, metallic selenium, thallium sulfide, cadmium sulfide, cadmiumselenide, lead sulfide as well as in general all the materials which areused in the well known photoresistive cells. Layers having a highsensitivity to the visible part of the spectrum may be used to reducethe loss of light transmitted across the lens, mirrors etc serving toform the optical image to be reproduced. Cadmium selenide and sulfidelayers are well adapted for the high speed production of copies fromsuccessive different light images because of the virtual instantaneousresponse of these layers to the light and to the dark.

In the arrangement of FIG. 1, a transparent electrode 1 is disposedbeneath a transparent backing material 2 to which is applied aphotoconductive layer 3. A light image comprising illuminated parts 5and low illuminated parts 4 is formed on the layer 3. Owing to thedifferences of light intensity between the parts 4 and 5, thephotoconductive layer is provided with a conductivity pattern includingconductive parts 5 and low conductive parts 4, respectively. It will beappreciated however that between the maximum and the minimumconductivities in the layer, any intermediate electrical conductivitymay be found in the photoconductive layer accordingly to thehalf-shadows of the light image to reproduce. In order to form the lightimage, for example, an objective 10 is located in front of thetransparent electrode 1, and beneath the objective 10, the document 11to be reproduced. Light sources 12 illuminate the document 11 thatreflects the light toward the objective 10 which projects it across theelectrode 1 and the transparent backing 2 on the photoconductive layer 3over which the optical image to be reproduced is formed. Document 11 maybe a sheet of paper carrying printed or typewritten matter, or aphotograph, for example, although other things may be phoused to formthe pattern 4, 5 such as, for example, X-

or gamma-rays; furthermore, any other means inducing in the layer 3 apattern of conductive parts 5 and low conductive parts 4 may be used toproduce electrographic images according to the present invention. If avisible light image is formed on layer 3, electrode 1 may consist, forexample, in a thin uniform layer'ofNesa, a high conductive transparentvarnish sold by Pittsburg Plate Glass Co, Pittsburg. The layer of Nesamay be affixed to a support of glass, for example. Moreover and forexample, a sheet of aluminium may constitute the transparent electrode 1when a X-rays image is formed on layer 3. As shown in FIG. 1, a thinuniform layer of developer powder 6 is disposed between layer 3 and animage carrier 7. The powder 6 may be applied to coat layer 3 or imagecarrier 7 or both, alternatively, for the uniform application of thepowder, classic spraying or cascading devices may be used, provided thata thin uniform layer of powder 6 is formed rather than a particularamount of grains. In order to develop electrographic images, an electricfield is generated between the electrode 1 and a second electrode 8 byapplying an electric voltage to terminals 9. Electrode 8 may be placedin contact with the image carrier 7 or, alternatively, an insulatinglayer (not shown) may be inserted between them. Under the influence ofan electric field, the layer of powder 6 is electrically distributedbetween the conductivity pattern 4, 5 and the image carrier 4. Whensubsequently, electrodes 1 and 8 are separated and the image carrier 7is detached from layer 3, a part of powder 6 will be found forming afirst electrographic image on the image carrier 7, while the remainingpart of the powder forms a secod electrographic image on layer 3, thetwo electrographic images being obtained in substantial configurationwith the optic image 4, 5.

According to one embodiment of the present invention an image carrier 7is used which has a uniform electric conductivity between the maximumand the minimum conductivities of the pattern 4, 5 of layer 3. A sheetof a conductive paper may be used as image carrier 7. Moreover, theimage carrier 7 may also consist in a thin uniform metallic layer on asheet of insulating material, as, for example, a sheet of mylar.Referring to this embodiment of the invention, FIG. 4 schematicallyshows two grains 113 and 113' of layer of powder 6, a part of layer 3and a part of the image carrier 7 having said intermediate conductivitybetween the conductivities of the parts 4 and 5 of layer 3. Depending onthe relative conductivities of the parts 4, 5 and 7, the contactconductance r between grains 113 and the illuminated part 5 of layer 3is higher than the contact conductance r between grain 113 and imagecarrier 7; the contact conductance r, between grain 113 and imagecarrier 7 is higher than the contact conductance r between grain 113 andthe low illuminated parts 4 of layer 3. Under the influence of theelectric field generated between electrodes'l and 8, each grain ofpowder 6 is electrically charged under the sign of that surface to whichthe contact conductance is the more, and thus it will be electricallyattracted away from this surface. For this reason, irrespectively of thedirection of the electric field, the powder 113 will electricallymigrate from the illuminate conductive part 5 towards the image carrier7, while powder 113 migrates from image carrier 7 towards the lowilluminated low conductive part 4. The electrographic image thus formedby powder 6 on the parts 4 of layer 3 will be termed positive uprightimage, and negative reversed image is called the electrographic imageformed on the image carrier 7 by the powder facing the parts 5 of layer3.

From the foregoing explanations it becomes apparent that, by using animage carrier 7 having said intermediate conductivity, the developmentof the electrographic images is irrespective of the minimum conductivityof layer 3; thus, according to the invention, a photoconductwvenon-insulating layer having a relatively high dark conductivity may beused.

Satisfactory continuous tone electrographic images may be produced byapplying an alternating or of an alternatively modulated electricvoltage to terminals 9, for example, from 1 to [O KV. In order todevelop electrographic images from the pattern of a photoconductivenon-insulating layer, an electric voltage ionizing the gap of air 13interposed between image carrier 7 and layer 3 may be advantageouslyapplied to terminals 9. To this end also a direct electric field may begenerated which is modulated by an alternating signal to produce copies.On the other hand, it will be appreciated that the best quality ofcontinuous tone electrographic images is obtained by using a layer 3which has a photoelectric linear character as for example a cadmiumsulfide layer.

In the arrangement shown in FIG. 2, a powder-coated photoconductivelayer 3 provided with a pattern ofilluminated conductive parts 5 and lowilluminated low conductive parts 4 is disposed under an electrode 108 inthe form of a grid. The coating powder 6 is insulated from the grid 108by a fluid dielectric consisting, for ex ample, of an air layer 107. Forexample, the grid 108 may be made of brass and have a mesh width ofabout 0.5 mm; the spacing between the grid 108 and layer 3 may be from0.5 to 5 mm, for example. A voltage generator (not shown) may beconnected to terminals 9 to create an electric field between thegrid-electrode 108 and a second electrode 101. The thin layer of powder6 is applied loosely-adhering to the layer 3. According with theexperience, the adherence of the powder may be improved by providing anelectrode 101 in the form of parallel wires so that the lines of forceof theelectric field strongly converge toward the electrode 101.

By generating an electric field between electrodes 101 and 108, thepowder is electrically charged and removed from the conductive parts 5,while the powder coating the low conductive parts 4 is neversufficiently charged to electrically overcome its adherence to layer 3and thus it develops a stable electrographic image thereon. The grainsof powder which are electrically attracted away from the parts 5 of thelayer 3 will pass through the fluid layer 107 and the grid-electrode108. The intensity of the electric field cannot exceed 3.3v/micron inthe layer of air 107 to avoid a sudden electric discharge betweenelectrode 108 and coating powder 6, which would reduce the electricfield serving to the development of the image. Instead of this, thequality of the electrographic images is improved by generating betweenelectrodes 101 and 108 an electric field having, in the air layer 107, agradient between 2.5 and 3.1 v/micron to obtain a silent ionizingdischarge in the air 107 simultaneously with the development of theelectrographic image; in this manner powder 6 will be electricallycharged from the slight conductive air 107 to adhere to the lowconductive parts 4, while the electric field remains sufficientlyintense, in the air 107, to electrically charge and remove the coatingpowder from the conductive parts 5. When a layer'3 affixed to aninsulating backing 2 is used a direct electric field may be generatedbetween electrodes 101 and 108.

Devices of FIGS. 1 and 2 serve to electrically photograph a document 11as well as three-dimensional objects, for example. Furthermore and forexample, as shown in FIG. 2, the layer 3 may move in the direction ofarrow 20 in the device simultaneously with document 11 in the directionindicated by the arrow, the latter having a synchroneous parallelmovement beneath objective 10. During the movement of document 11 and oflayer 3, the powder 106 drops from a container 30 on the layer 3 and itis uniformly distributed in a thin uniform layer 6 coating thephotoconductive layer 3. As shown in FIG. 2, the photoconductive layer 3may be affixed to a backing sheet 2 separated from the insulating layer102. Document 11, illuminated by light sources 12, moves in thedirection indicated by the arrow whereas the photoconductive layer 3 andits support 2 moves in opposite direction with a synchroneous movementcapable of immobilizing, in relation to the photoconductive layer, theoptic image formed on the latter. An electric field is created betweenelectrodes 101 and 108 and thus the powder 6 is drawn by electrode 108from the illuminated zones 5 of the photoconductive layer whereas itremains on the layer 3 over the dark zones 4 of the projected image.

According to one embodiment using the device illustrated in FIG. 2, analternating voltage is applied to terminals. The powder 6 thus receivesfrom the pattern 4,

5 alternating electric charges having different maximum values inproportion to the conductivities of said pattern and the most chargedpowder is electrically attracted through the grid-electrode 108.Referring to this method, FIG. 3 schematically shows two grains 113,113' of the powder 6 coating the photoconductive layer 3; by the letterb is indicated the equal force which retains the grains 113, 113' onlayer 3, this force I; may be due to the gravity, for instance. Becauseof the different illuminations of the parts and 4 of layer 3, thecontact conductance r between grains 113 and the illuminated part 5 ishigher than the contact conductance r bet-ween grain 113' and the lowilluminated part 5 of layer 3. By generating an alternating electricfield between electrodes 101 and 108 (FIG. 2), grains 113 and 113receive from layer 3 alternating charges having different maximum valuesaccording to the different contact conductances r and r under the actionof the field the charged grains 113, 113' are attracted away from layer3 by modulated electric forces having maximum values a and a insubstantial proportion to the contact conductance r and r respectively;the amplitude of the alternating voltage is then adjusted to apply tograin 113 the force a more intense that its adherence b to layer 3while, because of the alternating character of the charges, the electricforce a applied to grain 113' is never sufficiently intense to overcomethe adherence of this grain 113' to the low illuminated part 4 of layer3.

The electrographic image being obtained in a stable way, its goodquality is irrespective of a critical duration of the electric field.

According to a further embodiment using the device of FIG. 2, aphotoconductive layer 3 is used which is affixed to an insulatingbacking 2. By applying an alternatively modulated voltage to terminals9, the coating powder 6 receives from the pattern 4, 5 alternatingelectric charges having maximum values in proportion to theconductivities of said pattern 4, 5; the amplitude of the alternatingmodulation of the electric voltage is adjusted to electrically attractpowder 6 away from the illuminated parts 5 while the remaining powderdevelops a stable electrographic image on the low illuminated parts 4.Because of the insulation of the pattern 4, 5 from electrode 101,electric currents filterring through the low conductive parts 4 areavoided and thus a low frequency of the field is not critical in orderto produce stable images from a photoconductive noninsulating layer 3.This frequency may be as low as 50 Hz, for example. The amplitude of themodulated field is then adjusted to attract particles of powder havingsuccessive opposite polarities away from the most illuminated parts 5 ofthe layer, while the powder coating the least illuminated parts 4 isnever sufficiently charged to be removed; because of the oppositecharges of these grains, the removal of the powder may be prosecuted toelectrically remove all the powder coating the most illuminated parts 5of the layer 3 while the remaining part of the coating powder forms astable electrographic image of high density. This method is well adaptedto produce satisfactory electrographic images from a photoconductivelayer 3 having low differences in conductivity between its parts 4 and5. On the other hand, a direct voltage may be applied to terminals 9 toproduce stable images from a photoconductive non-insulating layer 3having high differences in conductivity.

For carrying out the invention as described with reference to FIGS. 2and 3, an apparatus of the type illustrated in FIG. 5 may be used. Thisapparatus comprises four rollers over which an endless belt 2 3 travelsin the direction of arrow 120. This endless belt is constituted by atransparent and flexible support 2 on to which is affixed aphotoconductive layer 3. A transparent dielectric plate 102 is made ofglass, for example, and it serves to guide the belt 2-3. The transparentelectrode 1 and the grid-electrode 108 are connected to the terminals 9of a voltage generator. A microfilm projector comprises a light source112, an objective 110 and a film unroller 140 of which the unrollingdirection is reversed that the endless belt 2-3, as indicated by arrowsand 120, respectively. In operation, the photoconductive layer 3 isaffixed to its flexible transparent support 2 is driven by rollers 115at a constant speed in synchronism with the movement of the projectorfilm 111. The light source 112 being lighted, the image is projected onlayer 3 through the transparent electrode 1 and the glass plate 102.Belt 2-3 moving in the direction of arrow 120, the developer powder 106of the container uniformly coats the photoconductive layer 3 and thethin uniform layer of powder 6 is driven by the upward movement of thelatter in the electric field generated between electrodes 1 and 108. Inorder to insure the adherence of powder 6 to the layer 3, a slightadheisve powder may be used, as for example, a powder 106 the grains ofwhich are coated with zinc stearate. Under the action of the electricfield, the powder coating illuminated parts 5 of layer 3 is electricallyattracted through the gridelectrode 108 and falls again in the container130, whie the powder coating low illuminated parts 4 forms a stableelectrographic image thereon. Thereafter, the powder image 104-105 istransferred on to the layer 3 by rollers 114 and 115'. Web 116 may be aweb of copy paper. If an excited photoconductive layer 3 is used whichis provided with a pattern 4, 5 having low differences in conductivity,an alternatively modulated voltage is applied to terminals 9. I

In the device of FIG. 1 it will be advantageous to use a developerpowder having an electric conductivity between the maximum and theminimum conductivities of the parts forming the pattern 4, 5 of layer 3.F urthermore, in the device of FIGS. 2 and 5, it will be expedient theuse of a developer powder having an electric conductivity about equal tothat of the least conductive parts of the pattern 4, 5. Although theexact conductivity of the powder is not critical in order to obtainelectrographic images of satisfactory quality.

In carrying out this invention a developer powder of charcoal has beenfound useful; alternatively, other developer powders, such as metallicor thermoplastic powders may be used. By way of example, a suitabledeveloper powder can be produced by oxidizing at a temperature of about700 C a commercial bronze to obtain a black powder containing copperbioxide as well as other metallic oxides; the powder is then passedthrough sieves to reduce the grains size between 2 and 25 microns. Theresulting powder can advantageously be coated with stearic acid or zincstearate or aluminium stearate, alternatively; such a treatment willrender the powder somewhat adhesive and give to its grains a very thininsulating coat which prevents electric discharges between contiguousparts of the layer of powder 6 during the application of the electricfield. Furthermore, after the formation of the images in the device ofFIG. 1, the grains of powder 6 conserve intense residual electriccharges because of their thin insulating coat and thus the obtainedelectrographic images will electrically adhere to the non-insulatingparts of layer 3 and to the uniformly conductive image carrier 4.

The conductivity of copper bioxide powder is generally between about 10and l Siemens/cm. It is moreover possible to use a commercial bronzecoloured powder; such powder has a conductivity from 10 to10"Siemens/cm.

Other types of developers, such as thermoplastic powders may be used as,for example, powders of polystyrene resins; these plastics materials maybe rendered conductive by mixing them with pure carbone, as it is wellknown in the art. Furthermore, an insulating thermoplastic powder may bemade conductive by coating its grains with a thin metallic layer, forexample.

While the method herein described and the apparatus used for carryingout thismethod into effect constitute preferred embodiments of theinvention, it is to be understood that the invention is not limited tothis precise method and apparatus, and changes may be made in eitherwithout departing from the scope of the invention which is defined inthe appended claims.

What I claim is:

l. A method for producing an electrographic image comprising the stepsof placing a layer'of electrically chargeable particles against aphotoconductive layer, exposing said photoconductive layer to a patternof radiation to form a conductivity pattern in said photoconductivelayer, generating across said layer of electrically chargeable particleand said photoconductive layer an alternatively modulated electric fieldthereby transferring alternating electric charges from said conductivitypattern to said layer of electrically chargeable particles whereby saidlayer of particles receives a pattern of greater and lesser alternatingcharges, said greater alternating charges removing a part of saidparticles while said lesser alternating charges maintain the remainingparticles in said layer of particles thereby developing a stableelectrographic image.

2. The method of claim 1 wherein said photoconductive layer is locatedbetween an insulating backing and said layer of electrically chargeableparticles and said alternatively modulated electric field is generatedacross said layer of electrically chargeable particles, saidphotoconductive layer and said insulating backing.

3. The method of claim 1 wherein said layer of electrically chargeableparticles is located between said photoconductive layer and aninsulating layer, and said alternatively modulated electric field isgenerated across said insulating layer, said layer of electricallychargeable particles and said photoconductive layer.

4. The method of claim 3 wherein said insulating layer is formed from afluid layer interposed between said photoconductive layer and agrid-shaped electrode, and said alternatively modulated electric fieldis generated between said photoconductive layer and said grid-electrode,whereby said removed part of the particles is definitively attractedacross said fluid and said grid-electrode away from said electric field.

5. The method of claim 1 wherein said layer of electrically chargeableparticles is located between said photoconductive layer and an imagecarrier whereby a second stable electrographic image is developed onsaid image carrier by said removed part of said particles.

6. A method for producing an electrographic image comprising the stepsof placing a layer of electrically chargeable particles against aphotoconductive layer, exposing said photoconductive layer to a patternof radiation to form in said photoconductive layer a conductivitypattern including maximum and minimum electric conductivities,sandwiching said layer of electrically chargeable particles between saidphotoconductive layer and an image carrier having a uniform electricconductivity between the maximum and minimum conductivities which areincluded in said conductivity pattern, generating across said imagecarrier, said layer of electrically chargeable particles andsaidphotoconductive layer an alternatively modulated electric field chargingsaid layer of electrically chargeable particles from said conductivitypattern and said image carrier simultaneously, said layer ofelectrically chargeable particles thereby receiving electric chargesattracting a part of said particles away from said image carrier todevelop a first stable electrographic image on said photoconductivelayer and opposite electric charges attracting the remainingparticlesaway from said photoconductive layer to develop a second stableelectrographic image on said image carrier.

7. The method of claim 6 wherein said photoconductive layer is locatedbetween an insulating backing and said layer of electrically chargeableparticles and said electric field is generated across said imagecarrier, said layer of electrically chargeable particles, saidphotoconductive layer and said insulating backing.

8. The method of claim 6 wherein said image carrier is located bewteensaid layer of electrically chargeable particles and an insulating layer,and said electric field is generated across said insulating layer, saidimage carrier, said layer of electrically chargeable particles and saidphotoconductive layer.

9. The method of claim 6 wherein the sandwich, formed by said layer ofelectrically chargeable particles disposed between said photoconductivelayer and said image carrier, is located between two insulating layers,and said electric field is generated across said two insulating layers,said image carrier, said layer of electrically chargeable particles andsaid photoconductive layer.

10. A method for producing an electrographic image comprising the stepsof coating a conductive image carrier with a layer of loosely-adheringelectrically chargeable particles, placing said coated image carrieragainst a photo-conductive layer so that said layer of electricallychargeable particles is sandwiched between said photoconductive layerand said conductive image carrier, exposing said photoconductive layerto a pattern of radiation to form a conductivity pattern in saidphotoconductive layer, generating across said conductive image carrier,said layer of electrically chargeable particles and said photoconductivelayer an alternatively modulated electric field so as to charge saidlayer of electrically chargeable particles from said photoconductivelayer and said conductive image carrier simultaneously, said layer ofelectrically chargeable particles thereby receiving a pattern of greaterand lesser alternating charges, said greater alternating chargesattracting a part of said particles away from said image carrier todevelop a first stable electrographic image on said photoconductivelayer while said lesser alternating charges maintain the remainingparticles on said image carrier thereby developing thereon second stableelectrographic image.

11. The method of claim wherein said photoconductive layer is locatedbetween'said layer of electrically chargeable particles and aninsulating backing, and said alternatively modulated electric field isgenerated across said conductive image carrier, said layer ofelectrically chargeable particles, said photoconductive layer and saidinsulating backing.

12. The method of claim 10, wherein said conductive image carrier islocated between said layer of electrically chargeable particles and aninsulating layer, and said alternatively modulated electric field isgenerated across said insulating layer, said conductive image carrier,said layer of electrically chargeable particles and said photoconductivelayer.

13. The method of claim 10, wherein the sandwich, formed by said layerof electrically chargeable particles disposed between said image carrierand said photoconductive layer, is located between two insulatinglayers, and said alternatively modulated electric field is generatedacross said two insulating layers, said image carrier, said layer ofelectrically chargeable particles and said photoconductive layer.

14. A method for producing an electrographic image comprising the stepsof affixing a photoconductive layer on an insulating backing material,coating said photoconductive layer with a layer of electricallychargeable particles, disposing said layer of electrically chargeableparticles between said photoconductive layer and an insulating layer,exposing said photoconductive layer to a pattern of radiation to form aconductivity pattern in said photoconductive layer, generating acrosssaid insulating layer, said layer of electrically chargeable particles,said photoconductive layer and said insulating backing material analternatively modulated electric field thereby transferring alternatingelectric charges from said conductivity pattern to said layer ofelectrically chargeable particles whereby said layer of particlesreceives a pattern of greater and lesser alternating charges, saidgreater alternating charges removing a part of said particles while saidlesser alternating charges maintain the remaining particles in saidlayer of particles thereby developing a stable electrographic image.

2. The method of claim 1 wherein said photo-conductive layer is locatedbetween an insulating backing and said layer of electrically chargeableparticles and said alternatively modulated electric field is generatedacross said layer of electrically chargeable particles, saidphotoconductive layer and said insulating backing.
 3. The method ofclaim 1 wherein said layer of electrically chargeable particles islocated between said photoconductive layer and an insulating Layer, andsaid alternatively modulated electric field is generated across saidinsulating layer, said layer of electrically chargeable particles andsaid photoconductive layer.
 4. The method of claim 3 wherein saidinsulating layer is formed from a fluid layer interposed between saidphotoconductive layer and a grid-shaped electrode, and saidalternatively modulated electric field is generated between saidphotoconductive layer and said grid-electrode, whereby said removed partof the particles is definitively attracted across said fluid and saidgrid-electrode away from said electric field.
 5. The method of claim 1wherein said layer of electrically chargeable particles is locatedbetween said photoconductive layer and an image carrier whereby a secondstable electrographic image is developed on said image carrier by saidremoved part of said particles.
 6. A method for producing anelectrographic image comprising the steps of placing a layer ofelectrically chargeable particles against a photoconductive layer,exposing said photoconductive layer to a pattern of radiation to form insaid photoconductive layer a conductivity pattern including maximum andminimum electric conductivities, sandwiching said layer of electricallychargeable particles between said photoconductive layer and an imagecarrier having a uniform electric conductivity between the maximum andminimum conductivities which are included in said conductivity pattern,generating across said image carrier, said layer of electricallychargeable particles and said photoconductive layer an alternativelymodulated electric field charging said layer of electrically chargeableparticles from said conductivity pattern and said image carriersimultaneously, said layer of electrically chargeable particles therebyreceiving electric charges attracting a part of said particles away fromsaid image carrier to develop a first stable electrographic image onsaid photoconductive layer and opposite electric charges attracting theremaining particles away from said photoconductive layer to develop asecond stable electrographic image on said image carrier.
 7. The methodof claim 6 wherein said photoconductive layer is located between aninsulating backing and said layer of electrically chargeable particlesand said electric field is generated across said image carrier, saidlayer of electrically chargeable particles, said photoconductive layerand said insulating backing.
 8. The method of claim 6 wherein said imagecarrier is located bewteen said layer of electrically chargeableparticles and an insulating layer, and said electric field is generatedacross said insulating layer, said image carrier, said layer ofelectrically chargeable particles and said photoconductive layer.
 9. Themethod of claim 6 wherein the sandwich, formed by said layer ofelectrically chargeable particles disposed between said photoconductivelayer and said image carrier, is located between two insulating layers,and said electric field is generated across said two insulating layers,said image carrier, said layer of electrically chargeable particles andsaid photoconductive layer.
 10. A method for producing an electrographicimage comprising the steps of coating a conductive image carrier with alayer of loosely-adhering electrically chargeable particles, placingsaid coated image carrier against a photo-conductive layer so that saidlayer of electrically chargeable particles is sandwiched between saidphotoconductive layer and said conductive image carrier, exposing saidphotoconductive layer to a pattern of radiation to form a conductivitypattern in said photoconductive layer, generating across said conductiveimage carrier, said layer of electrically chargeable particles and saidphotoconductive layer an alternatively modulated electric field so as tocharge said layer of electrically chargeable particles from saidphotoconductive layer and said conductive image carrier simultaneously,said layer of electrically chargeable partIcles thereby receiving apattern of greater and lesser alternating charges, said greateralternating charges attracting a part of said particles away from saidimage carrier to develop a first stable electrographic image on saidphotoconductive layer while said lesser alternating charges maintain theremaining particles on said image carrier thereby developing thereonsecond stable electrographic image.
 11. The method of claim 10 whereinsaid photoconductive layer is located between said layer of electricallychargeable particles and an insulating backing, and said alternativelymodulated electric field is generated across said conductive imagecarrier, said layer of electrically chargeable particles, saidphotoconductive layer and said insulating backing.
 12. The method ofclaim 10, wherein said conductive image carrier is located between saidlayer of electrically chargeable particles and an insulating layer, andsaid alternatively modulated electric field is generated across saidinsulating layer, said conductive image carrier, said layer ofelectrically chargeable particles and said photoconductive layer. 13.The method of claim 10, wherein the sandwich, formed by said layer ofelectrically chargeable particles disposed between said image carrierand said photoconductive layer, is located between two insulatinglayers, and said alternatively modulated electric field is generatedacross said two insulating layers, said image carrier, said layer ofelectrically chargeable particles and said photoconductive layer.
 14. Amethod for producing an electrographic image comprising the steps ofaffixing a photoconductive layer on an insulating backing material,coating said photoconductive layer with a layer of electricallychargeable particles, disposing said layer of electrically chargeableparticles between said photoconductive layer and an insulating layer,exposing said photoconductive layer to a pattern of radiation to form aconductivity pattern in said photoconductive layer, generating acrosssaid insulating layer, said layer of electrically chargeable particles,said photoconductive layer and said insulating backing material analternatively modulated electric field thereby transferring alternatingelectric charges from said conductivity pattern to said layer ofelectrically chargeable particles whereby said layer of particlesreceives a pattern of greater and lesser alternating charges, saidgreater alternating charges removing a part of said particles while saidlesser alternating charges maintain the remaining particles in saidlayer of particles thereby developing a stable electrographic image.